1. Field of the Invention
The present invention relates to tooling for a super-plastic forming tool used to form heated aluminum sheets.
2. Background Art
Conventional sheet metal forming processes generally used to form steel metal panels for vehicles and other applications are not easily adapted to the forming of aluminum sheets. Aluminum sheet metal has reduced formability. Aluminum sheets, when formed in conventional sheet metal forming processes, suffer from insufficient ductility in the metal and spring back. In addition, tooling costs for aluminum sheet metal forming tools may be increased as the result of added steps taken to compensate for the reduced formability of the aluminum sheets.
One approach to forming aluminum sheets that shows promise is super-plastic gas-pressure forming. In a super-plastic gas-pressure forming process, a single sided concave tool is provided. The tool is heated and a blank is clamped to the die. The sheet metal blank, after being heated, is formed by the application of gas pressure and may also be formed by drawing a vacuum in the concave die. The aluminum sheet is formed to the contour of the female die. To successfully form with the super-plastic gas-pressure forming process, the cavity must be sealed so that pressure applied to one side of the blank is not dissipated. The seal is normally established by providing a seal bead on the tooling that engages the periphery of the aluminum sheet.
While various seal bead geometries have been developed, the geometry of seal beads that were previously used to obtain a satisfactory seal suffered from the drawback of causing aluminum sheets to stick to the die making removal of the formed part more difficult.
One example of an early method of thermoforming metal is disclosed in U.S. Pat. No. 3,340,101. In the '101 patent, the periphery of the metal sheet is clamped by a clamping ring that holds it in place during the thermoforming process.
In U.S. Pat. No. 6,347,583, a seal bead for super-plastic forming of aluminum sheets is disclosed. The seal bead shapes disclosed in the '583 patent provide a gas tight seal suitable for stretch forming. The seal bead shape disclosed in the '583 patent is stated to limit deformation of the sheet so that the sheet does not stick to the bead or to the tool during the forming process. The seal bead disclosed in the '583 patent provides a cusp cross-sectional shape that is machined into the binder surface for engaging the periphery of the sheet material. The cusp, as disclosed, is formed by the intersection of two arcs so that the bead penetrates the sheet to provide a gas tight seal but with minimal contact area. While the '583 patent recognized the problem, the solution proposed in the '583 patent required a complex die bead shape that results in increased manufacturing costs. In addition, the seal bead geometry disclosed in the '583 patent fails to take advantage of the difference in the thermal expansion characteristics between the die and sheet to facilitate removal of the sheet from the die.
The above problems are addressed by applicants' invention as summarized below.